Copper CHA zeolite catalysts

ABSTRACT

Zeolite catalysts and systems and methods for preparing and using zeolite catalysts having the CHA crystal structure are disclosed. The catalysts can be used to remove nitrogen oxides from a gaseous medium across a broad temperature range and exhibit hydrothermal stable at high reaction temperatures. The zeolite catalysts include a zeolite carrier having a silica to alumina ratio from about 15:1 to about 256:1 and a copper to alumina ratio from about 0.25:1 to about 1:1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/480,360, filed Jun. 8, 2009, which is a divisional of U.S. patentapplication Ser. No. 12/038,423, filed on Feb. 27, 2008, now U.S. Pat.No. 7,601,662, issued Oct. 13, 2009, which claims the benefit ofpriority under 35 U.S.C. §119(e) to U.S. Provisional Application No.60/891,835, filed on Feb. 27, 2007, the contents of each of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments of the invention relate to zeolites that have the CHAcrystal structure, methods for their manufacture, and catalystscomprising such zeolites. More particularly, embodiments of theinvention pertain to copper CHA zeolite catalysts and methods for theirmanufacture and use in exhaust gas treatment systems.

BACKGROUND ART

Zeolites are aluminosilicate crystalline materials having rather uniformpore sizes which, depending upon the type of zeolite and the type andamount of cations included in the zeolite lattice, typically range fromabout 3 to 10 Angstroms in diameter. Both synthetic and natural zeolitesand their use in promoting certain reactions, including the selectivereduction of nitrogen oxides with ammonia in the presence of oxygen, arewell known in the art.

Metal-promoted zeolite catalysts including, among others, iron-promotedand copper-promoted zeolite catalysts, for the selective catalyticreduction of nitrogen oxides with ammonia are known. Iron-promotedzeolite beta has been an effective catalyst for the selective reductionof nitrogen oxides with ammonia. Unfortunately, it has been found thatunder harsh hydrothermal conditions, such as reduction of NOx from gasexhaust at temperatures exceeding 500° C., the activity of manymetal-promoted zeolites begins to decline. This decline in activity isbelieved to be due to destabilization of the zeolite such as bydealumination and consequent reduction of metal-containing catalyticsites within the zeolite. To maintain the overall activity of NOxreduction, increased levels of the iron-promoted zeolite catalyst mustbe provided. As the levels of the zeolite catalyst are increased toprovide adequate NOx removal, there is an obvious reduction in the costefficiency of the process for NOx removal as the costs of the catalystrise.

There is a desire to prepare materials which offer low temperature SCRactivity and/or improved hydrothermal durability over existing zeolites,for example, catalyst materials which are stable at temperatures up toat least about 650° C. and higher.

SUMMARY

Aspects of the invention are directed to zeolites that have the CHAcrystal structure (as defined by the International Zeolite Association),catalysts comprising such zeolites, and exhaust gas treatmentsincorporating such catalysts. The catalyst may be part of an exhaust gastreatment system used to treat exhaust gas streams, especially thoseemanating from gasoline or diesel engines.

One embodiment of the present invention pertains to copper CHA catalystsand their application in exhaust gas systems such as those designed toreduce nitrogen oxides. In specific embodiments, novel copper chabazitecatalysts are provided which exhibit improved NH₃ SCR of NOx. The copperchabazite catalysts made in accordance with one or more embodiments ofthe present invention provide a catalyst material which exhibitsexcellent hydrothermal stability and high catalytic activity over a widetemperature range. When compared with other zeolitic catalysts that findapplication in this field, such as Fe Beta zeolites, copper CHA catalystmaterials according to embodiments of the present invention offerimproved low temperature activity and hydrothermal stability.

One embodiment of the invention relates to a catalyst comprising azeolite having the CHA crystal structure and a mole ratio of silica toalumina greater than about 15 and an atomic ratio of copper to aluminumexceeding about 0.25. In a specific embodiment, the mole ratio of silicato alumina is from about 15 to about 256 and the atomic ratio of copperto aluminum is from about 0.25 to about 0.50. In a more specificembodiment, the mole ratio of silica to alumina is from about 25 toabout 40. In an even more specific embodiment, the mole ratio of silicato alumina is about 30. In one particular embodiment, the atomic ratioof copper to aluminum is from about 0.30 to about 0.50. In a specificembodiment, the atomic ratio of copper to aluminum is about 0.40. In aspecific embodiment, the mole ratio of silica to alumina is from about25 to about 40 and the atomic ratio of copper to aluminum is from about0.30 to about 0.50. In another specific embodiment, the silica toalumina is about 30 and the atomic ratio of copper to alumina is about0.40.

In a particular embodiment, the catalyst contains ion-exchanged copperand an amount of non-exchanged copper sufficient to maintain NOxconversion performance of the catalyst in an exhaust gas streamcontaining nitrogen oxides after hydrothermal aging of the catalyst. Inone embodiment, the NOx conversion performance of the catalyst at about200° C. after aging is at least 90% of the NOx conversion performance ofthe catalyst at about 200° C. prior to aging. In a particularembodiment, the catalyst contains at least about 2.00 weight percentcopper oxide.

In at least one embodiment, the catalyst is deposited on a honeycombsubstrate. In one or more embodiments, the honeycomb substrate comprisesa wall flow substrate. In other embodiments, the honeycomb substratecomprises a flow through substrate. In certain embodiments, at least aportion of the flow through substrate is coated with CuCHA adapted toreduce oxides of nitrogen contained in a gas stream flowing through thesubstrate. In a specific embodiment, at least a portion of the flowthrough substrate is coated with Pt and CuCHA adapted to oxidize ammoniain the exhaust gas stream.

In embodiments that utilize a wall flow substrate, at least a portion ofthe wall flow substrate is coated with CuCHA adapted to reduce oxides ofnitrogen contained in a gas stream flowing through the substrate. Inother embodiments, at least a portion of the wall flow substrate iscoated with Pt and CuCHA adapted to oxidize ammonia in the exhaust gasstream.

In a specific embodiment, a catalyst article comprises a honeycombsubstrate having a zeolite having the CHA crystal structure deposited onthe substrate, the zeolite having a mole ratio of silica to aluminagreater than about 15 and an atomic ratio of copper to aluminumexceeding about 0.25 and containing an amount of free copper exceedingion-exchanged copper. In one embodiment, the free copper is present inan amount sufficient to prevent hydrothermal degradation of the nitrogenoxide conversion of the catalyst. In one or more embodiments, the freecopper prevents hydrothermal degradation of the nitrogen oxideconversion of the catalyst upon hydrothermal aging. The catalyst mayfurther comprise a binder. In particular embodiments, the ion-exchangedcopper is exchanged using copper acetate.

Other aspects of the invention relate to exhaust gas treatment systemsincorporating catalysts of the type described above. Still other aspectsrelate to a process for the reduction of oxides of nitrogen contained ina gas stream in the presence of oxygen wherein said process comprisescontacting the gas stream with the catalyst described above.

Another aspect pertains to an exhaust gas treatment system comprising anexhaust gas stream containing NOx, and a catalyst described aboveeffective for selective catalytic reduction of at least one component ofNOx in the exhaust gas stream. Still another aspect pertains to anexhaust gas treatment system comprising an exhaust gas stream containingammonia and a catalyst as described above effective for destroying atleast a portion of the ammonia in the exhaust gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures for CuCHA prepared according to themethods of Example 1;

FIG. 1A is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures for CuCHA prepared according to themethods of Examples 1 and 1A;

FIG. 2 is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures, for CuCHA prepared according to themethods of Example 2;

FIG. 3 is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures for CuCHA prepared according to themethods of Example 3;

FIG. 4 is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures for CuCHA prepared according to themethods of Example 4;

FIG. 5 is a graph depicting effects of CO, propene, n-octane and wateron the CuCHA SCR activity at various temperatures;

FIG. 5A is a graph showing the amount of HCs that are stored, released,deposited as coke and burnt-off coke for a sample tested in accordancewith Example 12A;

FIG. 5B is a bar chart showing hydrocarbon performance of CuCHA comparedwith CuY and Fe beta zeolites in accordance with Example 12A;

FIG. 6 is a graph depicting emissions of NH₃, NOx (=NO+NO₂), N₂O, and N₂from the AMOX catalyst outlet, given as ppm on a nitrogen atom basisprepared and aged according to the method of Examples 13 and 14;

FIG. 7 is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures, for CuCHA prepared according to themethods of Example 16;

FIG. 8 is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures, for CuCHA prepared according to themethods of Example 17;

FIG. 9 is a graph depicting nitrogen oxides removal efficiency (%),ammonia consumption (%) and N₂O generated (ppm) of CuCHA catalyst as afunction of reaction temperatures for CuCHA prepared according to themethods of Example 18;

FIGS. 10A, 10B, and 10C are schematic depictions of three exemplaryembodiments of the emissions treatment system of the invention;

FIG. 11 is UV/VIS of Example 22 and 22A; and

FIG. 12 is ²⁷Al MAS NMR spectra of Example 22 and 22A, compared with CHAand aged CHA samples.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

In one embodiment of the invention, zeolites having the CHA structuresuch as chabazite are provided. In one or more embodiments, a zeolitehaving the CHA crystal structure and a mole ratio of silica to aluminagreater than about 15 and an atomic ratio of copper to aluminumexceeding about 0.25 is provided. In specific embodiments, the moleratio of silica to alumina is about 30 and the atomic ratio of copper toaluminum is about 0.40. Other zeolites having the CHA structure,include, but are not limited to SSZ-13, LZ-218, Linde D, Linde R, Phi,ZK-14, and ZYT-6.

Synthesis of the zeolites having the CHA structure can be carried outaccording to various techniques known in the art. For example, in atypical SSZ-13 synthesis, a source of silica, a source of alumina, andan organic directing agent are mixed under alkaline aqueous conditions.Typical silica sources include various types of fumed silica,precipitated silica, and colloidal silica, as well as silicon alkoxides.Typical alumina sources include boehmites, pseudo-boehmites, aluminumhydroxides, aluminum salts such as aluminum sulfate, and aluminumalkoxides. Sodium hydroxide is typically added to the reaction mixture,but is not required. A typical directing agent for this synthesis isadamantyltrimethylammonium hydroxide, although other amines and/orquaternary ammonium salts may be substituted or added to the latterdirecting agent. The reaction mixture is heated in a pressure vesselwith stirring to yield the crystalline SSZ-13 product. Typical reactiontemperatures are in the range of 150 and 180° C. Typical reaction timesare between 1 and 5 days.

At the conclusion of the reaction, the product is filtered and washedwith water. Alternatively, the product may be centrifuged. Organicadditives may be used to help with the handling and isolation of thesolid product. Spray-drying is an optional step in the processing of theproduct. The solid product is thermally treated in air or nitrogen.Alternatively, each gas treatment can be applied in various sequences,or mixtures of gases can be applied. Typical calcination temperaturesare in the 400° C. to 700° C. range.

CuCHA zeolite catalysts in accordance with one or more embodiments ofthe invention can be utilized in catalytic processes which involveoxidizing and/or hydrothermal conditions, for example in temperatures inexcess of about 600° C., for example, above about 800° C. and in thepresence of about 10% water vapor. More specifically, it has been foundthat CuCHA zeolite catalysts which have been prepared in accordance withembodiments of the invention have increased hydrothermal stabilitycompared to CuY and CuBeta zeolites. CuCHA zeolite catalysts prepared inaccordance with embodiments of the invention yield improved activity inthe selective catalytic reduction of NOx with ammonia, especially whenoperated under high temperatures of at least about 600° C., for example,about 800° C. and higher, and high water vapor environments of about 10%or more. CuCHA has high intrinsic activity that enables use of loweramounts of catalyst material, which in turn should reduce backpressureof honeycomb substrates coated with washcoats of CuCHA catalysts. In oneor more embodiments, hydrothermal aging refers to exposure of catalystto a temperature of about 800° C. in a high water vapor environments ofabout 10% or more, for at least about 5 to about 25 hours, and inspecific embodiments, up to about 50 hours.

Embodiments of this invention also pertain to a process for abatement ofNO in an exhaust gas stream generated by an internal combustion engineutilizing CuCHA zeolite catalysts having a mole ratio of silica toalumina greater than about 15 and an atomic ratio of copper to aluminumexceeding about 0.25. Other embodiments pertain to SCR catalystscomprising a CuCHA zeolite catalyst having a mole ratio of silica toalumina greater than about 15 and an atomic ratio of copper to aluminumexceeding about 0.25, and exhaust gas treatment systems incorporatingCuCHA zeolite catalysts. Still other embodiments pertain to ammoniaoxidation (AMOX) catalysts and exhaust gas treatment systemsincorporating AMOX catalyst comprising a CuCHA zeolite catalyst having amole ratio of silica to alumina greater than about 15 and an atomicratio of copper to aluminum exceeding about 0.25. According to one ormore embodiments, catalysts and systems utilize CuCHA catalysts havingion-exchanged copper and sufficient excess free copper to preventthermal degradation of the catalysts when operated under hightemperatures of at least about 600° C., for example, about 800° C. andhigher, and high water vapor environments of about 10% or more.

Experimentation has indicated that improved performance of catalysts inaccordance with embodiments of the invention is associated with Culoading. While Cu can be exchanged to increase the level of Cuassociated with the exchange sites in the structure of the zeolite, ithas been found that it is beneficial to leave non-exchanged Cu in saltform, for example, as CuSO₄ within the zeolite catalyst. Uponcalcination, the copper salt decomposes to CuO, which may be referred toherein as “free copper” or “soluble copper.” According to one or moreembodiments, this free Cu is both active and selective, resulting in lowN₂O formation when used in the treatment of a gas stream containingnitrogen oxides. Unexpectedly, this “free” Cu has been found to impartgreater stability in catalysts subjected to thermal aging attemperatures up to about 800° C.

While embodiments of the invention are not intended to be bound by aparticular principle, it is believed that the relatively small channelopenings of CHA do not permit large molecular weight hydrocarbons (HCs)typical of diesel fuel to enter and adsorb within the CuCHA structure.Unlike other zeolites like Beta or ZSM5, CHA catalysts preparedaccording to embodiments of the invention have a relatively low affinityfor adsorbing these large molecular weight HC species. This is abeneficial property for use in selective catalytic reduction (SCR)catalysts.

In systems that utilize an SCR downstream from a diesel oxidationcatalyst (DOC), the properties of the CuCHA catalysts provide one ormore beneficial results according to embodiments of the invention.During start-up and prolonged low temperature operation, the SCR only ora diesel oxidation catalyst (DOC) or DOC and catalyzed soot filter (CSF)upstream of the CuCHA SCR are not fully activated to oxidize the HCs. Inaccordance with one or more embodiments, because the CuCHA SCR catalystis not influenced by HCs at low temperature, it remains active over awider range of the low temperature operation window. According to one ormore embodiments, low temperature refers to temperatures about 250° C.and lower.

According to one or more embodiments, the CuCHA catalysts operate withina low temperature window. Over time in an exhaust gas treatment systemhaving a DOC pre-catalyst downstream from the engine followed by an SCRcatalyst and a CSF, or a DOC pre-catalyst upstream from a CSF and SCR,the DOC will tend to activate for both low temperature light-off and HCfuel burning. In such systems, it is beneficial if the SCR catalyst canmaintain its ability to operate at low temperatures. Since the oxidationcatalysts will lose their ability to oxidize NO to NO₂, it is useful toprovide an SCR catalyst that can treat NO as effectively as NO₂. CuCHAcatalysts produced in accordance with embodiments of the invention havethe ability to reduce NO with NH₃ at low temperatures. This attributecan be enhanced by the addition of non-exchanged Cu to the zeolitecatalyst.

According to embodiments of the invention, the SCR catalyst can be inthe form of self supporting catalyst particles or as a honeycombmonolith formed of the SCR catalyst composition. In one or moreembodiments of the invention however, the SCR catalyst composition isdisposed as a washcoat or as a combination of washcoats on a ceramic ormetallic substrate, for example a honeycomb flow through substrate.

In a specific embodiment of an emissions treatment system the SCRcatalyst is formed from a Cu exchanged CHA zeolite material having freecopper in addition to ion-exchanged copper.

When deposited on the honeycomb monolith substrates, such SCR catalystcompositions are deposited at a concentration of at least about 0.5g/in³, for example, about 1.3 g/in³ about 2.4 g/in³ or higher to ensurethat the desired NOx reduction is achieved and to secure adequatedurability of the catalyst over extended use.

The term “SCR” catalyst is used herein in a broader sense to mean aselective catalytic reduction in which a catalyzed reaction of nitrogenoxides with a reductant occurs to reduce the nitrogen oxides.“Reductant” or “reducing agent” is also broadly used herein to mean anychemical or compound tending to reduce NOx at elevated temperature. Inspecific embodiments, the reducing agent is ammonia, specifically anammonia precursor, i.e., urea and the SCR is a nitrogen reductant SCR.However, in accordance with a broader scope of the invention, thereductant could include fuel, particularly diesel fuel and fractionsthereof as well any hydrocarbon and oxygenated hydrocarbons collectivelyreferred to as an HC reductant.

Substrates

The catalyst compositions are disposed on a substrate. The substrate maybe any of those materials typically used for preparing catalysts, andwill usually comprise a ceramic or metal honeycomb structure. Anysuitable substrate may be employed, such as a monolithic substrate ofthe type having fine, parallel gas flow passages extending therethroughfrom an inlet or an outlet face of the substrate, such that passages areopen to fluid flow therethrough (referred to as honeycomb flow throughsubstrates). The passages, which are essentially straight paths fromtheir fluid inlet to their fluid outlet, are defined by walls on whichthe catalytic material is disposed as a washcoat so that the gasesflowing through the passages contact the catalytic material. The flowpassages of the monolithic substrate are thin-walled channels, which canbe of any suitable cross-sectional shape and size such as trapezoidal,rectangular, square, sinusoidal, hexagonal, oval, circular, etc. Suchstructures may contain from about 60 to about 400 or more gas inletopenings (i.e., cells) per square inch of cross section.

The substrate can also be a wall-flow filter substrate, where thechannels are alternately blocked, allowing a gaseous stream entering thechannels from one direction (inlet direction), to flow through thechannel walls and exit from the channels from the other direction(outlet direction). AMOX and/or SCR catalyst composition can be coatedon the flow through or wall-flow filter. If a wall flow substrate isutilized, the resulting system will be able to remove particulate matteralong with gaseous pollutants. The wall-flow filter substrate can bemade from materials commonly known in the art, such as cordierite,aluminum titanate or silicon carbide. It will be understood that theloading of the catalytic composition on a wall flow substrate willdepend on substrate properties such as porosity and wall thickness, andtypically will be lower than loading on a flow through substrate.

The ceramic substrate may be made of any suitable refractory material,e.g., cordierite, cordierite-alumina, silicon nitride, zircon mullite,spodumene, alumina-silica magnesia, zircon silicate, sillimanite, amagnesium silicate, zircon, petalite, alpha-alumina, an aluminosilicateand the like.

The substrates useful for the catalysts of embodiments of the presentinvention may also be metallic in nature and be composed of one or moremetals or metal alloys. The metallic substrates may be employed invarious shapes such as corrugated sheet or monolithic form. Suitablemetallic supports include the heat resistant metals and metal alloyssuch as titanium and stainless steel as well as other alloys in whichiron is a substantial or major component. Such alloys may contain one ormore of nickel, chromium and/or aluminum, and the total amount of thesemetals may advantageously comprise at least 15 wt. % of the alloy, e.g.,10-25 wt. % of chromium, 3-8 wt. % of aluminum and up to 20 wt. % ofnickel. The alloys may also contain small or trace amounts of one ormore other metals such as manganese, copper, vanadium, titanium and thelike. The surface or the metal substrates may be oxidized at hightemperatures, e.g., 1000° C. and higher, to improve the resistance tocorrosion of the alloys by forming an oxide layer on the surfaces thesubstrates. Such high temperature-induced oxidation may enhance theadherence of the refractory metal oxide support and catalyticallypromoting metal components to the substrate.

In alternative embodiments, one or both of the CuCHA catalystcompositions may be deposited on an open cell foam substrate. Suchsubstrates are well known in the art, and are typically formed ofrefractory ceramic or metallic materials.

Washcoat Preparation

According to one or more embodiments, washcoats of CuCHA can be preparedusing a binder. According to one or more embodiments use of a ZrO₂binder derived from a suitable precursor such as zirconyl acetate or anyother suitable zirconium precursor such as zirconyl nitrate. In oneembodiment, zirconyl acetate binder provides a catalytic coating thatremains homogeneous and intact after thermal aging, for example, whenthe catalyst is exposed to high temperatures of at least about 600° C.,for example, about 800° C. and higher, and high water vapor environmentsof about 10% or more. Keeping the washcoat intact is beneficial becauseloose or free coating could plug the downstream CSF causing thebackpressure to increase.

According to one or more embodiments, CuCHA catalysts can be used as anammonia oxidation catalyst. Such AMOX catalysts are useful in exhaustgas treatment systems including an SCR catalyst. As discussed incommonly assigned U.S. Pat. No. 5,516,497, the entire content of whichis incorporated herein by reference, a gaseous stream containing oxygen,nitrogen oxides and ammonia can be sequentially passed through first andsecond catalysts, the first catalyst favoring reduction of nitrogenoxides and the second catalyst favoring the oxidation or otherdecomposition of excess ammonia. As described in U.S. Pat. No.5,516,497, the first catalysts can be a SCR catalyst comprising azeolite and the second catalyst can be an AMOX catalyst comprising azeolite.

As is known in the art, to reduce the emissions of nitrogen oxides fromflue and exhaust gases, ammonia is added to the gaseous streamcontaining the nitrogen oxides and the gaseous stream is then contactedwith a suitable catalyst at elevated temperatures in order to catalyzethe reduction of nitrogen oxides with ammonia. Such gaseous streams, forexample, the products of combustion of an internal combustion engine orof a gas-fueled or oil-fueled turbine engine, often inherently alsocontain substantial amounts of oxygen. A typical exhaust gas of aturbine engine contains from about 2 to 15 volume percent oxygen andfrom about 20 to 500 volume parts per million nitrogen oxides, thelatter normally comprising a mixture of NO and NO₂. Usually, there issufficient oxygen present in the gaseous stream to oxidize residualammonia, even when an excess over the stoichiometric amount of ammoniarequired to reduce all the nitrogen oxides present is employed. However,in cases where a very large excess over the stoichiometric amount ofammonia is utilized, or wherein the gaseous stream to be treated islacking or low in oxygen content, an oxygen-containing gas, usually air,may be introduced between the first catalyst zone and the secondcatalyst zone, in order to insure that adequate oxygen is present in thesecond catalyst zone for the oxidation of residual or excess ammonia.

Metal-promoted zeolites have been used to promote the reaction ofammonia with nitrogen oxides to form nitrogen and H₂O selectively overthe competing reaction of oxygen and ammonia. The catalyzed reaction ofammonia and nitrogen oxides is therefore sometimes referred to as theselective catalytic reduction (“SCR”) of nitrogen oxides or, assometimes herein, simply as the “SCR process”. Theoretically, it wouldbe desirable in the SCR process to provide ammonia in excess of thestoichiometric amount required to react completely with the nitrogenoxides present, both to favor driving the reaction to completion and tohelp overcome inadequate mixing of the ammonia in the gaseous stream.However, in practice, significant excess ammonia over suchstoichiometric amount is normally not provided because the discharge ofunreacted ammonia from the catalyst to the atmosphere would itselfengender an air pollution problem. Such discharge of unreacted ammoniacan occur even in cases where ammonia is present only in astoichiometric or sub-stoichiometric amount, as a result of incompletereaction and/or poor mixing of the ammonia in the gaseous stream,resulting in the formation therein of channels of high ammoniaconcentration. Such channeling is of particular concern when utilizingcatalysts comprising monolithic honeycomb-type carriers comprisingrefractory bodies having a plurality of fine, parallel gas flow pathsextending therethrough because, unlike the case of beds of particulatecatalyst, there is no opportunity for gas mixing between channels.

According to embodiments of the present invention CuCHA catalysts can beformulated to favor either (1) the SCR process, i.e., the reduction ofnitrogen oxides with ammonia to form nitrogen and H₂O, or (2) theoxidation of ammonia with oxygen to form nitrogen and H₂O, theselectivity of the catalyst being tailored by controlling the Cu contentof the zeolite. U.S. Pat. No. 5,516,497 teaches iron and copper loadinglevels on zeolites other than copper CHA to obtain selectivity for anSCR reaction and selectivity of the catalyst for the oxidation ofammonia by oxygen at the expense of the SCR process, thereby improvingammonia removal. In accordance with embodiments of the invention, CuCHAcopper loading can be tailored to obtain selectivity for SCR reactionsand oxidation of ammonia by oxygen and to provide exhaust gas treatmentsystems utilizing both types of catalyst.

The above principles are utilized by providing a staged or two-zonecatalyst in which a first catalyst zone with copper loading on azeolite, that promotes SCR followed by a second catalyst zone comprisinga zeolite having thereon copper loading and/or a precious metalcomponent that promotes oxidation of ammonia. The resultant catalystcomposition thus has a first (upstream) zone which favors the reductionof nitrogen oxides with ammonia, and a second (downstream) zone whichfavors the oxidation of ammonia. In this way, when ammonia is present inexcess of the stoichiometric amount, whether throughout the flow crosssection of the gaseous stream being treated or in localized channels ofhigh ammonia concentration, the oxidation of residual ammonia by oxygenis favored by the downstream or second catalyst zone. The quantity ofammonia in the gaseous stream discharged from the catalyst is therebyreduced or eliminated. The first zone and the second zones can be on asingle catalyst substrate or as separate substrates.

It has been demonstrated that a CuCHA washcoat containing a preciousmetal, for example, Pt, provides an AMOX catalyst. It is expected thatnot only was ammonia in gas flowing through the catalyst destroyed, butthere was continued removal of NOx by conversion to N₂. In a specificembodiment, the zeolite has a ratio of SiO₂/Al₂O₃ from about 15 to about256, and an Al/M ratio between 2 and 10, wherein M represents the totalCu and precious metal. In one embodiment, the precious metal comprisesplatinum and the platinum content is between 0.02% and 1.0% by weight ofthe catalyst, and the part loading is from about 0.5 to about 5 g/in³.

According to one or more embodiments of the invention, CuCHA SCRcatalysts can be disposed on a wall-flow filter or catalyzed sootfilter. CuCHA washcoats can be coated on a porous filter to provide forsoot combustion, SCR and AMOX functions.

In one or more embodiments of the present invention, the catalystcomprises a precious metal component, i.e., a platinum group metalcomponent. For example, as noted above, AMOX catalysts typically includea platinum component. Suitable precious metal components includeplatinum, palladium, rhodium and mixtures thereof. The severalcomponents (for example, CuCHA and precious metal component) of thecatalyst material may be applied to the refractory carrier member, i.e.,the substrate, as a mixture of two or more components or as individualcomponents in sequential steps in a manner which will be readilyapparent to those skilled in the art of catalyst manufacture. Asdescribed above and in the examples, a typical method of manufacturing acatalyst according to an embodiment of the present invention is toprovide the catalyst material as a coating or layer of washcoat on thewalls of the gas-flow passages of a suitable carrier member. This may beaccomplished by impregnating a fine particulate refractory metal oxidesupport material, e.g., gamma alumina, with one or more catalytic metalcomponents such as a precious metal, i.e., platinum group, compound orother noble metals or base metals, drying and calcining the impregnatedsupport particles and forming an aqueous slurry of these particles.Particles of the bulk copper chabazite may be included in the slurry.Activated alumina may be thermally stabilized before the catalyticcomponents are dispersed thereon, as is well known in the art, byimpregnating it with, e.g., a solution of a soluble salt of barium,lanthanum, zirconium, rare earth metal or other suitable stabilizerprecursor, and thereafter drying (e.g., at 110° C. for one hour) andcalcining (e.g., at 550° C. for one hour) the impregnated activatedalumina to form a stabilizing metal oxide dispersed onto the alumina.Base metal catalysts may optionally also have been impregnated into theactivated alumina, for example, by impregnating a solution of a basemetal nitrate into the alumina particles and calcining to provide a basemetal oxide dispersed in the alumina particles.

The carrier may then be immersed into the slurry of impregnatedactivated alumina and excess slurry removed to provide a thin coating ofthe slurry on the walls of the gas-flow passages of the carrier. Thecoated carrier is then dried and calcined to provide an adherent coatingof the catalytic component and, optionally, the copper CHA material, tothe walls of the passages thereof. One or more additional layers may beprovided to the carrier. After each layer is applied, or after a thenumber of desired layers is applied, the carrier is then dried andcalcined to provide a finished catalyst member in accordance with oneembodiment of the present invention.

Alternatively, the alumina or other support particles impregnated withthe precious metal or base metal component may be mixed with bulk orsupported particles of the copper chabazite material in an aqueousslurry, and this mixed slurry of catalytic component particles andcopper chabazite material particles may be applied as a coating to thewalls of the gas-flow passages of the carrier.

In use, the exhaust gas stream can be contacted with a catalyst preparedin accordance with embodiments of the present invention. For example,the CuCHA catalysts made in accordance with embodiments of the presentinvention are well suited to treat the exhaust of engines, includingdiesel engines.

Without intending to limit the invention in any manner, embodiments ofthe present invention will be more fully described by the followingexamples.

EXAMPLE 1

A CuCHA powder catalyst was prepared by mixing 100 g of NH₄ ⁺-form CHA,having a silica/alumina mole ratio of 30, with 400 mL of a copper(II)sulfate solution of 1.0 M. The pH was adjusted to 3.5 with nitric acid.An ion-exchange reaction between the NH₄ ⁺-form CHA and the copper ionswas carried out by agitating the slurry at 80° C. for 1 hour. Theresulting mixture was then filtered, washed with 800 mL of deionizedwater in three portions until the filtrate was clear and colorless,which indicated that substantially no soluble or free copper remained inthe sample, and the washed sample was dried at 90° C. The above processincluding the ion-exchange, filtering, washing and drying was repeatedonce.

The resulting CuCHA product was then calcined at 640° C. in air for 6hours. The obtained CuCHA catalyst comprised CuO at 2.41% by weight, asdetermined by ICP analysis. A CuCHA slurry was prepared by mixing 90 gof CuCHA, as described above, with 215 mL of deionized water. Themixture was ball-milled. 15.8 g of zirconium acetate in dilute aceticacid (containing 30% ZrO₂) was added into the slurry with agitation.

The slurry was coated onto 1″D×3″L cellular ceramic cores, having a celldensity of 400 cpsi (cells per square inch) and a wall thickness of 6.5mil. The coated cores were dried at 110° C. for 3 hours and calcined at400° C. for 1 hour. The coating process was repeated once to obtain atarget washcoat loading of 2.4 g/in³.

Nitrogen oxides selective catalytic reduction (SCR) efficiency andselectivity of a fresh catalyst core was measured by adding a feed gasmixture of 500 ppm of NO, 500 ppm of NH₃, 10% O₂, 5% H₂O, balanced withN₂ to a steady state reactor containing a 1″D×3″L catalyst core. Thereaction was carried at a space velocity of 80,000 hr⁻¹ across a 150° C.to 460° C. temperature range.

Hydrothermal stability of the catalyst was measured by hydrothermalaging of the catalyst core in the presence of 10% H₂O at 800° C. for 50hours, followed by measurement of the nitrogen oxides SCR efficiency andselectivity by the same process as outlined above for the SCR evaluationon a fresh catalyst core.

FIG. 1 is graph showing the NOx conversion and N₂O make or formationversus temperature for this sample. These results are summarized inTable 1. This sample, which did not contain soluble copper prior tocalcination as indicated by the color of the filtrate described above,did not show enhanced resistance to thermal aging.

EXAMPLE 1A

To the coating slurry of Example 1 was added copper sulphatepentahydrate to bring up the total CuO level to 3.2%. The slurry wascoated onto monolith and aged and tested for SCR NO as outlined abovefor Example 1, except that the monolith was calcined at 640° C. Thecatalytic performance was compared with Example 1 in FIG. 1A. Theaddition of copper sulphate into the coating slurry significantlyimproved the hydrothermal stability and low temperature activity.

EXAMPLE 2

A CuCHA powder catalyst was prepared by mixing 17 Kg of NH₄ ⁺-form CHA,having a silica/alumina mole ratio of 30, with 68 L of a copper(II)sulfate solution of 1.0 M. The pH was adjusted to 3.5 with nitric acid.An ion-exchange reaction between the NH₄ ⁺-form CHA and the copper ionswas carried out by agitating the slurry at 80° C. for 1 hour. Theresulting mixture was then filtered and air-dried. The above processincluding the ion-exchange and filtering was repeated once. Then the wetfilter cake was reslurried into 40 L deionized water followed byfiltering and drying at 90° C. The resulting CuCHA product was thencalcined at 640° C. in air for 6 hours. The obtained CuCHA catalystcomprised CuO at 2.75% by weight.

The slurry preparation, coating and SCR NO_(x) evaluation were the sameas outlined above for Example 1. This example contained free copper, andexhibited improved hydrothermal stability compared with Example 1.

EXAMPLE 3

CuCHA catalyst comprising 3.36% CuO by weight was prepared by the sameprocess as that in Example 2 followed by an incipient wetnessimpregnation.

Using the procedure in Example 2, 134 grams of CuCHA at 3.11% CuO byweight was prepared. To this material, was added a copper sulfatesolution comprised of 1.64 g of copper sulfate pentahydrate and 105 mLof deionized water. The impregnated sample was dried at 90° C. andcalcined at 640° C. for 6 hours.

The slurry preparation, coating and SCR NO evaluation are the same asoutlined above for Example 1. As shown in FIG. 3, the sample containingmore non-exchanged copper exhibited higher low temperature activity inaddition to hydrothermal stability.

EXAMPLE 4

CuCHA catalyst comprising 3.85% CuO by weight was prepared by anincipient wetness impregnation process only. A copper sulfate solutioncomprised of 18.3 g of copper sulfate pentahydrate and 168 mL ofdeionized water was impregnated onto 140 g of NH₄ ⁺-form CHA, having asilica/alumina mole ratio of 30. The impregnated sample was then driedat 90° C. and calcined at 640° C. for 6 hours.

The slurry preparation, coating and SCR NO evaluation are the same asoutlined above for Example 1. As shown in FIG. 4, Example 4 exhibited adecline in performance between 350° C. and 450° C. after hydrothermalaging.

EXAMPLE 5

CuCHA catalyst comprising 1.94% CuO by weight was prepared by the sameprocess as that in Example 1, except that this sample was prepared by asingle ion-exchange.

The slurry preparation, coating and SCR NO evaluation are the same asoutlined above for Example 1, except that the hydrothermal stability wasnot measured.

EXAMPLE 6

A CuCHA powder catalyst was prepared by mixing 0.2 g of NH₄ ⁺-form CHA,having a silica/alumina mole ratio of 15, with 16 mL of a copper(II)sulfate solution of 25 mM. An ion-exchange reaction between the NH₄⁺-form CHA and the copper ions was carried out by agitating the slurryat 80° C. for 1 hour. The resulting mixture was then filtered, washedwith deionized water and dried at 90° C. The above process including theion-exchange, filtering, washing and drying was repeated once. Theresulting CuCHA product was then calcined at 540° C. in air for 16hours. The obtained CuCHA catalyst comprised CuO at 4.57% by weight.

The catalyst powder was hydrothermally aged in the presence of 10% H₂Oat 800° C. for 50 hours, followed by measurement of the nitrogen oxidesSCR efficiency.

Catalyst performance was evaluated using a microchannel catalyticreactor containing a bed of approximately 12.6 mm³ of catalyst. The flowrate (standard temperature and pressure) of 500 cc/min of reactants,consisting of 500 ppm NO_(x), 500 ppm NH₃, 10% O₂, 5% H₂O, balanced withHe, plus 25 cc/min steam was passed over the bed at various temperatures(200, 250, 300, 350, 400, 450 and 500° C.) to determine the reactivityof the catalyst. Conversion of NO_(x) was determined by 100*(NO_(x)fed—NO_(x) out)/(NO_(x) fed) using a mass spectral analyzer.

EXAMPLE 7

CuCHA powder catalyst comprising 2.94% CuO by weight was prepared by thesame process as that in Example 6, including ion-exchange, filtering,washing, drying, calcinations and hydrothermal aging, except that thesilica/alumina mole ratio was 30 and that the ion-exchange process wasrepeated 4 times.

The SCR NO evaluation is the same as outlined above for Example 6.

EXAMPLE 8

CuCHA powder catalyst comprising 0.45% CuO by weight was prepared by thesame process as that in Example 6, including ion-exchange, filtering,washing, drying, calcinations and hydrothermal aging, except that thesilica/alumina mole ratio was 50.

The SCR NO evaluation is the same as outlined above for Example 6.

EXAMPLE 9

A CuCHA powder catalyst was prepared by mixing 15.0 g of NH₄ ⁺-form CHA,having a silica/alumina mole ratio of 256, with 61 mL of a copper(II)sulfate solution of 0.64 M. An ion-exchange reaction between the NH₄⁺-form CHA and the copper ions was carried out by agitating the slurryat 80° C. for 1 hour. The resulting mixture was then filtered, washedwith deionized water and dried at 90° C. The above process including theion-exchange, filtering, washing and drying was repeated 4 times. Theresulting CuCHA product was then calcined at 540° C. in air for 16hours. The obtained CuCHA catalyst comprised CuO at 2.63% by weight.

The hydrothermal aging and SCR NO_(x) evaluation was the same asoutlined above for Example 6.

COMPARATIVE EXAMPLE 10

A Cu/Y zeolite powder catalyst was prepared having silica/alumina moleratio of 5 as described further below.

A Cu/Y powder catalyst was prepared by mixing 500 g of NH₄ ⁺-formZeolite Y, having a silica/alumina mole ratio of ˜5, with 2500 mL of acopper(II) sulfate solution of 0.1 M. The pH was between 2.9 and 3.3. Anion-exchange reaction between the NH₄ ⁺-form Y zeolite and the copperions was carried out by agitating the slurry at 80° C. for 1 hour. Theresulting mixture was then filtered, washed with deionized water anddried at 90° C. The above process including the ion-exchange, filtering,washing and drying was repeated for a total of 5 exchanges where pH wassimilar to above. The resulting Cu Zeolite Y product was then calcinedat 640° C. in air for 16 hours. The obtained Cu Zeolite Y catalystcomprised CuO at 4.60% by weight.

The Cu/Y slurry was prepared by mixing 200 g of Cu/Y, as describedabove, with 400 mL of deionized water. The mixture was milled by passingtwice through an Eigermill to obtain a slurry which comprised 90%particles smaller than 8 μm. 8.7 g of zirconium acetate in dilute aceticacid (containing 30% ZrO₂) was added into the slurry with agitation.

The slurry was coated onto 1″D×3″L cellular ceramic cores, having a celldensity of 400 cpsi (cells per square inch) and a wall thickness of 6.5mil. Two coats were required to obtain a target washcoat loading of 1.6g/in³. The coated cores were dried at 90° C. for 3 hours, and the coreswere calcined at 450° C. for 1 hour after the second drying step.

The hydrothermal aging and SCR evaluation are the same as outlined inExample 1, except aging at was performed 750° C. for 25 hours.

COMPARATIVE EXAMPLE 11

A Cu/Beta powder catalyst was prepared having silica/alumina mole ratiois 35 using a procedure similar to the sample prepared in EXAMPLE 10.The hydrothermal aging and SCR evaluation are the same as outlined inExample 1.

A summary of the data for Examples 1-5 and Comparative Examples 10-11 iscontained in Table 1 below.

TABLE 1 Cu/Al NO_(x) conversion (%) N₂O make, ppm Atomic 210° C., 210°C., 460° C., 460° C., 460° C., 460° C., Example ratio CuO % fresh agedfresh aged fresh aged 1 0.30 2.41 75 43 95 82 0.8 5.3 2 0.33 2.75 62 5990 83 3.1 9.3 3 0.38 3.36 74 70 91 81 2.7 10.5 4 0.44 3.85 76 60 88 723.5 14.2 5 0.24 1.94 50 30 95 75 0.2 5.0 10  0.23 4.6 43 42 99 96 26 5111  0.36 2.5 92 23 84 53 10 9.4 12  0.46 3.7 75 78 89 80 5.4 11.7 1A0.40 3.2 61 82 11.3

Table 1 indicates that Example 3 exhibited the best combination of lowtemperature activity, high temperature activity and showed littledegradation due to hydrothermal aging.

Table 2 shows the normalized NOx conversion for Examples 6-9, whichcontained varying SiO₂/Al₂O₃ Mole ratios and Cu/Al Atomic ratios.Example 7 exhibited the best performance. While the performance ofExamples 6, 8 and 9 was not optimal, it is to be noted that each of theExamples was aged at a rather high temperature of 800° C. Not allcatalysts will experience such high temperatures, and it is believedthat samples aged at lower temperatures would exhibit acceptableperformance at a wider acceptable silica/alumina ratio. For example, inan exhaust gas treatment system having an SCR catalyst downstream of acatalyzed soot filter, the SCR would typically be exposed to hightemperatures, e.g., exceeding about 700° C. If the SCR is disposed onthe CSF, the SCR may experience temperatures as high as about 800° C.,or higher. According to embodiments of the present invention, greaterflexibility in locating a catalyst such as an SCR catalyst in an exhaustgas treatment system is provided due to the CuCHA catalysts whichexhibit improved hydrothermal stability compared with other types ofzeolite materials. Samples having a range of silica to alumina ratiobetween about 15 and 256 which experience operational temperatures belowabout 800° C. would be expected to exhibit acceptable low temperatureNOx conversion. Thus, according to embodiments of the invention, silicato alumina ratios of about 15 to about 256 are within the scope of theinvention, however, narrower ranges having a lower range endpoint ofabout 10, 20, about 25 and about 30 and a higher range endpoint of 150,100, 75, 50 and 40 are within the scope of the invention.

TABLE 2 Cu/Al NO_(x) conversion, SiO₂/Al₂O₃ Atomic aged, normalizedExample Mole ratio CuO % ratio 200° C. 250° C. 300° C. 6 15 4.57 0.300.34 0.61 0.81 7 30 2.94 0.36 1.00 1.00 0.98 8 50 0.45 0.089 0.39 0.541.00 9 256 2.63 2.6 0.10 0.70 0.88

EXAMPLE 12 Cucha Inhibition Study

The samples tested in this Example were prepared as follows. A CuCHApowder catalyst was prepared by mixing 250 g of NH₄ ⁺-form CHA, having asilica/alumina mole ratio of 30, with 2.0 L of a copper(II) sulphatesolution of 0.1 M. The pH was adjusted to 3.0 to 3.4 with nitric acid.An ion-exchange reaction between the NH₄ ⁺-form CHA and the copper ionswas carried out by agitating the slurry at 80° C. for 1 hour. Theresulting mixture was then filtered, washed with deionized water anddried at 90° C. The above process including the ion-exchange, filtering,washing and drying was repeated for a total of 5 times. The resultingCuCHA product was then calcined at 640° C. in air for 16 hours. Theobtained CuCHA catalyst comprised CuO at 3.68% by weight.

The impact of CO, propene, n-octane and water on the CuCHA SCR activityat temperatures 170, 200, 250, 300 and 350° C. was investigated. Thecatalyst cores were tested in a simulated diesel exhaust mixture. Themain gas concentrations were as follows: 500 ppm NO, 500 ppm NH₃, 10%CO₂, 10% O₂. The following components were added sequentially toinvestigate the effect on the NOx conversion: 5% H₂O, 300 ppm C₃H₆ asCl, 600 ppm C₃H₆ as Cl, 100 ppm Octane as Cl and 500 ppm CO. The spacevelocity of the experiments was set to 142,000 h⁻¹. The reaction wasallowed to reach steady state at temperature points of 170° C., 200° C.,250° C., 300° C. and 350° C. and the subsequent conversions andcomponent interactions were recorded. Gas analysis of NO, NO₂, N₂O, NH₃,CO₂, CO, C₃H₆ and H₂O was performed using an MKS 2030 MultiGas FTIRrunning at 0.5 cm⁻¹ resolution.

The results are summarized in FIG. 5. At low temperatures 170° C. and200° C., water was the main inhibitor, high level of propen at 200 ppm(600 ppm Cl) was slightly inhibiting at 200 C, 100 ppm propene (300 ppmCl), CO, and n-octane had no impact. At temperatures higher than 250°C., water was observed to be a promoter. None of the components testedwere inhibiting the NOx conversion at 250° C., on the contrary they wereall promoters. At 300° C., CO and n-octane promoted the SCR NOx, whereas600 ppm Cl propene inhibited the reaction. At 350° C., only 600 ppm Clpropene had minor inhibition, and the other components all had positiveeffect. This performance is believed to be better than the performanceof other Cu-promoted SCR catalysts that use medium and large porezeolites, for example, beta zeolites. SCR catalysts are known to besusceptible to transient poisoning by long chain hydrocarbons, which canfill the pores with coke. These tests show that the small pore CuCHAzeolite did not exhibit this problem.

EXAMPLE 12A

HC Storage/Release Test:

Gases and Apparatus:

A catalyst core of CuCHA coated on a ceramic monolith (400 cpsi/6 mil)presenting a cross section of 144 open cells and 1″ length was firstaged for 50 h at 800 C in 10% H₂O, 10% O2, balance nitrogen.Subsequently, the catalyst was placed in a laboratory reactor. Thecatalyst was exposed to a gas mixture comprising 4% H₂O, 14% O₂, 100 ppmNO, balance N₂ and heated to 100° C. After temperature stabilization at100° C., a blend of toluene and octane was added via mass flowcontroller so as to achieve a target concentration of 100 ppm Cl asoctane and 100 ppm Cl as toluene at a total space velocity of 104 kh⁻¹.The effluent gas was led over an afterburner which was comprised of aPt/alumina based oxidation catalyst and kept at a constant temperatureof 600° C. Any hydrocarbon emissions including partial oxidationproducts and CO that might be formed over the CuCHA catalyst will beoxidized into CO₂ when passed over the afterburner. The CO₂ effluentfrom the afterburner is monitored by an IR CO₂ analyzer. In parallel, aslip stream of the effluent from the CuCHA catalyst bypassing theafterburner has been analyzed by a FID-HC analyzer.

Test Protocol:

After the stabilization of the CuCHA catalyst at 100° C. in a mixture of4% H₂O, 14% O₂, 100 ppm NO, balance N₂, the hydrocarbon blend of octaneand toluene was introduced. During 10 min the catalyst temperature waskept at 100° C. During this period, HCs are stored over the catalystwhich leads to a CO₂ afterburner out signal below the HC inletconcentration. After the storage period, the temperature is raisedlinearly from 100° C. to 600° C. at a ramp of 20° C./min. The CO₂afterburner signal increases sharply which is due to a release of storedof HCs from the catalyst. Upon completion of the desorption, the CO₂signal returns to the baseline value (=feed gas concentration). As thetemperature rises, a small decrease of the afterburner out CO₂ below thefeed gas level indicates a second type of HC removal which is due to thedeposition of carbonaceous deposits formed from toluene and octane overthe catalyst. As the temperature increases further any carbonaceousdeposits formed will burn off and cause an elevated CO₂ afterburner outsignal. After the burn off of carbonaceous deposits is completed, theCO₂ afterburner signal will eventually return to its baseline value.

Data Analysis:

The CO₂ afterburner signal was evaluated quantitatively in order todetermine the amount of HCs that are stored, released, deposited as cokeand burnt-off coke. The corresponding intersections of the afterburnerout CO₂ trace shown in FIG. 5A with the HC feed gas concentration wereused as integration limits. For the example of CuCHA these integrationlimits were approximately between 0 and 800 s for the storage, between800 s and 1000 s for the release, between 1000 s and 1400 s for thecoking, respectively. The amount of HCs that were stored, released,deposited as coke and subsequently burnt-off are expressed as mg HCbased on the average C:H ratio of the feed stream HCs.

Results:

This experiment was carried out with Cu—Y (after aging for 25 h @ 750 Cin 10% H₂O, 10% O₂, balance N₂) and Fe-Beta (after aging for 50 h at800° C. in 10% H₂O, 10% O₂, balance N₂) SCR catalysts of the same volumeunder the same conditions. In the case of CuCHA, there appears to bevery little coking and consequently there is no noticeable burn-offsignal. The results are graphed in FIG. 5B. It is evident that the CuCHAcatalyst stores the least amount of HCs of which most is released as HCsand little is deposited as coke. The Cu—Y catalyst on the contrary didform a substantial amount of carbonaceous deposits in the temperaturesrange from about 200° C. to 450° C. Part of the built up coke issubsequently burnt-off at higher temperatures.

EXAMPLE 13 Preparation of AMOX Catalyst

An ammonia oxidation catalyst comprising a CuCHA was prepared as inExample 12 and having a copper content of 3.68% measured as CuO, andSiO₂/Al₂O₃ ratio of 30. This material was coated onto a standardmonolithic cordierite support, having a square-cell geometry of 400cells/in³, to provide a total loading of 2.40 g/in³ based on monolithbulk volume. This pre-coated monolith was then dipped into a solution ofa platinum-containing precursor (a platinum hydroxy amine complex) tofully and uniformly distribute the platinum precursor on the part. Thepart was dried at 110° C. and then calcined at 450° C. for one hour.This provided a platinum loading on the part of 4.3 g/ft³ based onmonolith bulk volume. Thus the catalyst had the following composition:3.68% CuO+0.10% Pt supported on CuCHA, coated on standard cordierite400/6 support at total part loading of about 2.4 g/in³. The Al:Cu:Ptatomic ratio in the present catalyst is about 190:90:1. The Al/M ratio(M=Cu+Pt) is equal to about 2.1.

EXAMPLE 14 Testing of Samples of Example 13

Ammonia removal efficiency and oxidation product selectivities ofhydrothermally-aged AMOx catalyst cores prepared as described in Example13 were measured by adding a feed gas mixture of 500 ppm of NH₃, 10% O₂,5% H₂O, balanced with N₂ (as air) to a steady state reactor containing a3.0 inch long square-cylindrical catalyst core with a facial crosssection containing 144 open cells. The reaction was carried out at aspace velocity of 100,000 hr⁻¹ across a 150° C. to 460° C. temperaturerange. Hydrothermal aging conditions are 10 hours at 700° C. with 10%H₂O in air. FIG. 6 is a graph showing emissions compared with those froma hydrothermally-aged sample of CuCHA. The data show 1) the highlyselective NH₃ conversion to N₂ catalyzed by the CuCHA catalyst in theabsence of Pt impregnation, and 2) that the NH₃ conversion can bedramatically enhanced by inclusion of the platinum component withoutcompromising the high N₂ selectivity. The latter is significant in thatthe prior art shows that platinum as a metallic gauze or supported onother oxides or zeolitic supports is generally selective for productionof N₂O or NO_(x).

EXAMPLE 15

Comparison of the CuCHA formulation on a flow through substrate and awall flow filter at comparable loadings. A wall flow filter was coatedwith the same catalyst as the flow through catalyst carrier of Example 3and the two samples measure to compare their catalytic activity.

A CuCHA slurry was prepared by mixing 90 g of CuCHA, as described above,with 215 mL of deionized water. The mixture was ball-milled for 11 hoursto obtain a slurry which comprised 90% particles smaller than 10 μm.15.8 g of zirconium acetate in dilute acetic acid (containing 30% ZrO₂)was added into the slurry with agitation.

The slurry was coated onto 1″D×6″L cellular ceramic wall flow filtercores, having a cell density of 300 cpsi (cells per square inch) and awall thickness of 12 mil. The coated cores were dried at 120° C. for 3hours and calcined at 540° C. for 1 hour. The coating process wasrepeated once to obtain a target washcoat loading of 2.0 g/in³.

Nitrogen oxides selective catalytic reduction (SCR) efficiency andselectivity of a fresh catalyst core was measured by adding a feed gasmixture of 500 ppm of NO, 500 ppm of NH₃, 10% O₂, 5% H₂O, balanced withN₂ to a steady state reactor containing a 1″D×6″L catalyst core. Thereaction was carried at a space velocity of 40,000 hr⁻¹ across a 150° C.to 400° C. temperature range.

Hydrothermal stability of the catalyst was measured by hydrothermalaging of the catalyst core in the presence of 10% H₂O at 750° C. for 25hours, followed by measurement of the nitrogen oxides SCR efficiency andselectivity by the same process as outlined above for the SCR evaluationon a fresh catalyst core.

Table 3 below shows the comparison of the hydrothermally aged SCRperformance of the CuCHA coated on a filter versus the CuCHA coated on aflow through catalyst carrier.

TABLE 3 SCR performance comparison (% conversion) of filter and flowthrough substrates N₂O make Sample Temp NO NO₂ NOx NH₃ (ppm) (degreesC.) CuCHA on Flow through, aged 50 H @ 800 C. w/10% water 74.6 83.5 75.076.9 8.4 211 96.3 95.6 96.2 93.9 9.2 255 97.6 97.5 97.6 97.3 7.6 30982.7 36.5 81.0 98.1 12.3 441 CuCHA on filter, aged 25 H @ 750 C. w/10%water 74.7 81.5 75.1 76.0 8.8 207 96.4 96.1 96.4 96.5 9.9 255 98.6 97.798.5 96.8 8.7 304 96.2 90.7 95.9 98.7 8.2 352 91.1 62.4 89.8 99.4 11.7400

In spite of some differences in exact experimental detail, thecomparison clearly supports the equivalence of the catalytic performanceof CuCHA on the filter core and the flow through monolith catalyst.

EXAMPLE 16

An NH₄ ⁺-CHA slurry was prepared by mixing 608 g of NH₄ ⁺-CHA, having asilica/alumina mole ratio of 30, with 796 mL of deionized water. Themixture was milled using a Netzsch Mill to obtain a slurry whichcomprised 90% particles smaller than 8.4 μm. 106 g of zirconium acetatein dilute acetic acid (containing 30% ZrO₂) was added into the slurrywith agitation.

The slurry was coated onto 1″D×3″L cellular ceramic cores, having a celldensity of 400 cpsi and a wall thickness of 6.5 mil. The coated coreswere dried at 110° C. for 3 hours. The coating process was repeated onceto obtain a target washcoat loading of 2.4 g/in³.

This pre-coated monolith was then dipped into a 0.25M solution of copperacetate for 5 minutes at room temperature. The core was gently blownwith an air gun and dried at 110° C. for 3 hours and then calcined at400° C. for 1 hour. This provided a CuO loading on CHA of 2.72 wt.%based on the CHA weight on monolith.

The SCR NOx evaluation of the fresh catalyst was the same as outlinedfor Example 1. Hydrothermal stability of the catalyst was measured byhydrothermal aging of the catalyst core in the presence of 10% steam at850° C. for 6 hrs, followed by measurement of the SCR NOx efficiency asoutlined for the fresh catalyst.

FIG. 7 is graph showing the NOx conversion and N₂O formation versustemperature for this sample.

EXAMPLE 17

12.1 g of copper acetate monohydrate was dissolved in 420 g deionizedwater, then 141 g of NH₄ ⁺-CHA, having a silica/alumina mole ratio of30, was added in. The mixture was milled using a Netzsch Mill to obtaina slurry which comprised 90% particles smaller than 3.5 μm.

The slurry was coated onto 1″D×3″L cellular ceramic cores, having a celldensity of 400 cpsi and a wall thickness of 6.5 mil. The coated coreswere dried at 110° C. for 3 hours. The coating process was repeatedtwice to obtain a target washcoat loading of 2.4 g/in³. The coated coreswere then calcined at 400° C. for 1 hour. This provides a CuO loading onCHA of 3.3 wt.%.

The SCR NOx evaluation of the fresh catalyst was the same as outlinedfor Example 1. Hydrothermal stability of the catalyst was measured byhydrothermal aging of the catalyst core in the presence of 10% steam at850° C. for 6 hrs, followed by measurement of the SCR NOx efficiency asoutlined for the fresh catalyst.

FIG. 8 is graph showing the NOx conversion and N₂O formation versustemperature for this sample.

EXAMPLE 18

A CuCHA powder catalyst was prepared by ion-exchange with copperacetate. A 0.40 M of copper (II) acetate monohydrate solution wasprepared by dissolving 89.8 g of the copper salt in 1.125 L of deionizedwater at 70° C. 300 g of NH₄ ⁺-form CHA was then added to this solution.An ion-exchange reaction between the NH4⁺-form CHA and the copper ionswas carried out by agitating the slurry at 70° C. for 1 hour. The pH wasbetween 4.8 and 4.5 during the reaction. The resulting mixture was thenfiltered, washed until the filtrate had a conductivity of <200 μcm⁻¹,which indicated that substantially no soluble or free copper remained inthe sample, and the washed sample was dried at 90° C. The obtained CuCHAcatalyst comprised CuO at 3.06% by weight and Na₂O at 140 ppm.

The slurry preparation, coating and SCR NO evaluation were the same asoutlined above for Example 1. As shown in FIG. 9, Example 18 exhibitedthe same SCR performance as Example 3 that was prepared by twiceion-exchanges with copper sulphate plus an incipient wetnessimpregnation.

EXAMPLE 19

CuCHA catalyst comprising 2.99% CuO by weight was prepared by the sameprocess as that in Example 18, except that this sample was prepared in0.30 M Cu solution.

EXAMPLE 20

CuCHA catalyst comprising 2.69% CuO by weight was prepared by the sameprocess as that in Example 18, except that the ion-exchange wasprocessed at 45° C.

EXAMPLE 21

CuCHA catalyst comprising 2.51% CuO by weight was prepared by the sameprocess as that in Example 19, except that the ion-exchange wasprocessed at 45° C.

The Cu loadings of Examples 18-21 are compared with that of Example 1 inTable 4. We see that copper acetate is more efficient than coppersulphate to provide desired Cu loading with a low concentration ofcopper solution at lower reaction temperature.

TABLE 4 Cu²⁺ Example Cu salt Conc., M Reaction T, ° C. CuO wt. % 1 Cusulphate 1.0 80 2.41 18 Cu acetate 0.40 70 3.06 19 Cu acetate 0.30 702.99 20 Cu acetate 0.40 45 2.69 21 Cu acetate 0.30 45 2.51

EXAMPLE 22 Hydrothermal Aging and Chemical Analysis of Example 2

The Cu/CHA powder prepared in Example 2 was hydrothermally aged in thepresence of 10% H₂O in air at 800° C. for 48 hours. The analyzedmaterial from Example 2 is labeled Example 22 in FIGS. 11 and 12 andTables 5 and 6. The hydrothermally aged sample is labeled Example 22A inTables 5 and 6 and FIGS. 11 and 12.

The X-ray powder diffraction patterns were determined by standardtechniques. Generator settings are 45 kV and 40 mA. The diffractometeroptics consists of a variable divergence slit, incident beam sollerslits, a receiving slit, a graphite monochromater, and a scintillationcounter using Bragg-Brentano parafocusing geometry. The d-spacings werecalculated from the lattice parameters of a=13.58 and c=14.76 Å forExample 22 and a=13.56 and c=14.75 Å for Example 22A. The latticeparameters were determined by scanning the sample with LaB6 mixed in asan internal standard. The data range was 15-38.5 degrees two theta usinga step size of 0.01 and counting for 5 seconds. The resulting patternwas run through profile refinement in JADE software. The LaB6 latticeparameters were kept constant at 5.169 A to compensate for sampledisplacement errors. Table 5 shows the X-ray powder diffraction linesfor Example 22 and Example 22A. The CHA crystalline structure retainedafter 800° C. 48 hours steam aging.

TABLE 5 Example 22 Example 22A 2-Theta d(Å) I(%) 2-Theta d(Å) I(%) 9.639.201 100%  9.62 9.189 100%  13.02 6.793 37%  13.04 6.782 36%  14.156.252 8% 14.17 6.247 7% 16.21 5.465 28%  16.23 5.457 26%  18.01 4.92132%  18.03 4.917 30%  19.28 4.600 3% 19.30 4.595 3% 20.85 4.258 89% 20.88 4.251 82%  22.29 3.985 4% 22.31 3.981 4% 22.65 3.922 5% 22.693.916 4% 23.33 3.809 8% 23.37 3.804 7% 25.27 3.521 41%  25.29 3.519 38% 26.22 3.397 24%  26.26 3.391 23%  27.98 3.186 5% 28.03 3.181 5% 28.533.126 6% 28.56 3.123 5% 29.91 2.985 3% 29.96 2.980 3% 30.98 2.885 57% 31.03 2.880 53%  31.21 2.864 17%  31.23 2.862 17%  31.48 2.840 28% 31.51 2.837 26%  31.99 2.795 4% 32.04 2.792 4% 32.75 2.733 3% 32.802.728 3% 33.73 2.655 2% 33.78 2.651 2% 33.95 2.639 4% 33.98 2.637 4%34.92 2.568 13%  34.98 2.563 12%  35.38 2.535 3% 35.43 2.531 2% 36.502.460 9% 36.54 2.457 8% 38.72 2.324 2% 38.78 2.320 1% 38.90 2.313 1%38.93 2.312 1% 39.13 2.300 2% 39.18 2.297 2% 39.56 2.276 1% 39.62 2.2731% 39.78 2.264 2% 39.84 2.261 2%

UV/VIS diffuse reflectance spectra expressed by F(R) were collectedusing a diffuse reflectance attachment with an integrating and referencesphere coated with BaSO₄ inside a Cary 300 UV-Vis spectrometer. TheUV/VIS of Example 22 and 22A are shown in FIG. 11.

Table 6 lists the ²⁹Si MAS NMR (Magic Angle Spinning Nuclear MagneticResonance) data and the calculated framework Si/Al atomic ratio ofExample 22 and 22A. The data for the CHA and the 800° C., 48 hours, 10%steam-aged CHA are also included for comparison. The data indicate thata degree of de-alumination takes place upon aging of both CHA and Cu/CHAsamples. However, the Cu/CHA sample undergoes much less de-aluminationupon aging. It is also observed that the Cu-exchange process itselfslightly alters the framework Si/Al atomic ratio from 15 to 17.

FIG. 12 shows the ²⁷Al MAS NMR (Magic Angle Spinning Nuclear MagneticResonance) spectra of Example 22 and 22A, as well as the CHA and agedCHA samples. The spectra indicate that some of the tetrahedral Alspecies are converted to penta- and octa-coordinated species uponCu-exchange. The spectra strongly support that the Cu/CHA sampleundergoes much less de-alumination upon aging than the CHA sample.

TABLE 6 Intensity % Si(0Al)- Si(0Al)- Si(1Al)- Si(1Al)- Sample 114 ppm111 ppm 105 ppm 101 ppm Si/Al CHA 2 71 16 11 15 Aged CHA 0 95 1 4 82Example 22 2 75 19 5 17 Example 22A 4 85 11 <1 34

Exemplary embodiments of emission treatment systems are shown in FIGS.10A, 10B and 10C. One embodiment of the inventive emissions treatmentsystem denoted as 11A is schematically depicted in FIG. 10A. Theexhaust, containing gaseous pollutants (including unburned hydrocarbons,carbon monoxide and NOx) and particulate matter, is conveyed from theengine 19 to a position downstream in the exhaust system where areductant, i.e., ammonia or an ammonia-precursor, is added to theexhaust stream. The reductant is injected as a spray via a nozzle (notshown) into the exhaust stream. Aqueous urea shown on one line 25 canserve as the ammonia precursor which can be mixed with air on anotherline 26 in a mixing station 24. Valve 23 can be used to meter preciseamounts of aqueous urea which are converted in the exhaust stream toammonia.

The exhaust stream with the added ammonia is conveyed to the SCRcatalyst substrate 12 (also referred to herein including the claims as“the first substrate”) containing CuCHA in accordance with one or moreembodiments. On passing through the first substrate 12, the NOxcomponent of the exhaust stream is converted through the selectivecatalytic reduction of NOx with NH₃ to N₂ and H₂O. In addition, excessNH₃ that emerges from the inlet zone can be converted through oxidationby a downstream ammonia oxidation catalyst (not shown) also containingCuCHA to convert the ammonia to N₂ and H₂O. The first substrate istypically a flow through monolith substrate.

An alternative embodiment of the emissions treatment system, denoted as11B is depicted in FIG. 10B which contains a second substrate 27interposed between the NH₃ injector and the first substrate 12. In thisembodiment, the second substrate is coated with an SCR catalystcomposition which may be the same composition as is used to coat thefirst substrate 12 or a different composition. An advantageous featureof this embodiment is that the SCR catalyst compositions that are usedto coat the substrate can be selected to optimize NOx conversion for theoperating conditions characteristic of that site along the exhaustsystem. For example, the second substrate can be coated with an SCRcatalyst composition that is better suited for higher operatingtemperatures experienced in upstream segments of the exhaust system,while another SCR composition can be used to coat the first substrate(i.e., the inlet zone of the first substrate) that is better suited tocooler exhaust temperature which are experienced in downstream segmentsof the exhaust system.

In the embodiment depicted in FIG. 10B, the second substrate 27 caneither be a honeycomb flow through substrate, an open cell foamsubstrate or a honeycomb wall flow substrate. In configurations of thisembodiment where the second substrate is a wall flow substrate or a highefficiency open cell foam filter, the system can remove greater than 80%of the particulate matter including the soot fraction and the SOF. AnSCR-coated wall flow substrate and its utility in the reduction of NOxand particulate matter have been described, for instance, in co-pendingU.S. patent application Ser. No. 10/634,659, filed Aug. 5, 2003, thedisclosure of which is hereby incorporated by reference.

In some applications it may be advantageous to include an oxidationcatalyst upstream of the site of ammonia/ammonia precursor injection.For instance, in the embodiment depicted in FIG. 10C an oxidationcatalyst is disposed on a catalyst substrate 34. The emissions treatmentsystem 11C is provided with the first substrate 12 and optionallyincludes a second substrate 27. In this embodiment, the exhaust streamis first conveyed to the catalyst substrate 34 where at least some ofthe gaseous hydrocarbons, CO and particulate matter are combusted toinnocuous components. In addition, a significant fraction of the NO ofthe NOx component of the exhaust is converted to NO₂. Higher proportionsof NO₂ in the NOx component facilitate the reduction of NOx to N₂ andH₂O on the SCR catalyst(s) located downstream. It will be appreciatedthat in the embodiment shown in FIG. 10C, the first substrate 12 couldbe a catalyzed soot filter, and the SCR catalyst could be disposed onthe catalyzed soot filter. In an alternative embodiment, the secondsubstrate 27 comprising an SCR catalyst may be located upstream fromcatalyst substrate 34.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover modifications and variationsof this invention provided they come within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A SCR catalyst comprising an aluminosilicatezeolite having the CHA crystal structure and a mole ratio of silica toalumina from about 10 to about 150 and an atomic ratio of copper toaluminum from about 0.25 to about 1, the catalyst effective to promotethe reaction of ammonia with nitrogen oxides to form nitrogen and H₂Oselectively, wherein the catalyst is in a form of a honeycomb monolithformed of the SCR catalyst.
 2. The catalyst of claim 1, wherein the moleratio of silica to alumina is from about 15 to about
 100. 3. Thecatalyst of claim 1, wherein the mole ratio of silica to alumina is fromabout 15 to about 50 and the atomic ratio of copper to aluminum is fromabout 0.25 to about 0.5.
 4. A staged catalyst comprising analuminosilicate zeolite having the CHA crystal structure and a moleratio of silica to alumina from about 10 to about 150 and an atomicratio of copper to aluminum from about 0.25 to about 1, the catalysthaving a first catalyst zone with copper loading on the zeolite tailoredto promote an SCR reaction of ammonia with nitrogen oxides to formnitrogen and H₂O selectively followed by a second catalyst zonecomprising a zeolite having the CHA crystal structure and thereon acopper loading tailored to promote oxidation of ammonia.
 5. The stagedcatalyst of claim 4, wherein the second catalyst zone comprises aplatinum group metal.
 6. The staged catalyst of claim 4, wherein thezones are on a single honeycomb substrate.
 7. The staged catalyst ofclaim 4, wherein the zones are on a separate honeycomb substrate.